Controlled potential coulometry: the application of a secondary reaction to the determination of plutonium and uranium at a solid electrode

A method is described for the simultaneous determination of plutonium and uranium in mixed oxides by controlled potential coulometry at a gold working electrode in two stages: first a coulometric oxidation, at 0.73 V vs. a silver/silver chloride electrode, of Pu(III) and U(IV) to Pu(IV) and U(VI) by a combination of a direct electrode reaction and a secondary chemical reaction proceeding concurrently, and secondly, a coulometric reduction at 0.33 V of Pu(IV) to Pu(III), leaving uranium as U(VI). The determination is carried out in a mixture of sulphuric and nitric acids, and Ti(III) is used to reduce plutonium and uranium to Pu(III) and U(IV) before electrolysis. The precision (3sigma) of Pu:U ratio results obtained from mixtures containing about 30% and 2% plutonium was 0.5% and 1-5% respectively. The effect of experimental variables on the time taken to complete the coulometric determination is discussed.     

Constant-current coulometric determination of uranium in the pure metal

A new method is proposed for the highly precise and accurate constant-current coulometry of uranium in high-purity uranium. Precisely weighed amounts of uranium and pure iron are dissolved in 7 M sulfuric acid containing some hydrogen peroxide (40% $\text{v}\text{v}$). The solution is quantitatively transferred to the coulometric cell by rinsing with 1 M H₂SO₄, saturated with cerium(III) sulfate. The first step is the quantitative electro-chemical reduction to U(IV), Fe(II) and Ce(III) on a gold gauze electrode at constant current (100 mA) until evolution of hydrogen is observed. The hydrogen is then removed by flushing the solution with very pure nitrogen until the potential of a platinum gauze electrode reaches a constant value. Oxidation on the gold gauze electrode is carried out under precisely controlled constant current; after the quantitative oxidation of U(IV) to U(VI) and Fe(II) to Fe(III), and crossing the end-point, this end-point is determined very precisely potentiometrically through back-titration by successive current injections of 10 mA during 1 s. The method was tested on a NBS reference material, uranium (NBS 960).     

Electrochemistry of uranium in sodium chloroaluminate melts

The electrochemical behaviour of uranium has been studied in basic, NaCl-saturated NaAlCl₄ melts at 175°C. Solutions of UO₃ exhibit two oxidation/reduction waves (cyclic voltammetry). The first wave corresponds to the U(VI)/U(IV) redox couple and is irreversible (slow electron transfer). The second wave corresponds to the deposition and stripping of an insoluble U(III) compound (U(IV)/U(III)). Solutions of UO₂ or UCl₄ and U(IV) solutions prepared by exhaustive electrolysis of UO₃ behave identically. The cyclic voltammograms of U(IV) solutions are the same as those of UO₃, but they show additional anodic peaks. Analysis of the peak currents (cyclic voltammetry), the limiting currents (pulse polarography) and the non-linear log i-t curves (anodic controlled potential coulometry) leads to the conclusion that uranium (IV) in the basic chloroaluminate melt exists as two different species in slow equilibrium with one another, of which only one species can be oxidized to U(VI). E.m.f. measurements of U(VI)-U(IV) mixtures indicate that the electron transfer process involves the formation of an intermediate U(V) species in a disproportionation equilibrium.     

Differential controlled-potential coulometry: application to the determination of uranium

Differential controlled-potential coulometry has been investigated as a high-precision analytical technique using the reduction of uranium (VI) in 0.5 M sulphuric acid at a mercury cathode. A relative standard deviation of 0.009% was obtained with 30–120 mg samples of uranium and the method was applied to the comparison of uranium standards.     

Synergistic extraction and separation studies of uranium(VI)

A method is described for the extractive separation and spectrophotometric determination of uranium(VI) from an aqueous solution of pH 5.0–7.0 using benzoylacetone (bzac) and pyridine (py) dissolved in toluene as extractants. The extracted species are UO₂(bzac(₂·2py. The method provides separation of uranium(VI) from lanthanum(III), samarium(III), neodymium(III), cerium(III) and thorium(IV). The method is precise, accurate, fast and selective.     

Uranium oxide clusters by laser desorption ionization during MALDI-TOF MS analysis of uranium(VI)

Laser desorption ionization (LDI) mode of matrix-assisted laser desorptionionization time-of-flight mass spectrometry (MALDI-TOFMS) analysis of uranium(VI)leads to the formation of uranium oxides clusters, as with fast atom bombardment(FAB). Different uranium clusters than those with FAB were observed. Threedifferent families of formula (UO₂)_x O_y ²⁺, and two of formula (UO₂)_x O_y ²⁺ were found.     

Cation-exchange behaviour of uranium(vi) on amberlite ir-120 : Separation from mixtures

Quantitative studies are reported on the cation-exchange behaviour of uranium(VI) at the milligram level with Amberlite IR-120. Hydrochloric, nitric, sulphuric, perchloric, acetic and citric acids were tested as cluants; 200–300 ml of 2 N hydrochloric, nitric or sulphuric acid suffice for quantitative elution of 17 mg of uranium(VI) from a 1.4 cm X 14 5 cm bed The efficiency of the elutmg agents is discussed in terms of their elution constants Uranium is separated from thorium by selective elution, from zirconium, cerium(III), copper and nickel by converting the latter into suitable anionic complexes and from phosphate just by passing the mixture through the cation exchanger.     

Coulometric determination of uranium with a platinum working electrode

Experimental conditions have been established which enable uranium to be determined coulometrically by the reduction of uranium(VI) to uranium(IV) at a platinum working electrode, by controlled-potential or controlled-potential-limit techniques. The procedure has been used successfully as a subsidiary method in the routine determination of uranium in pure uranyl nitrate solutions. The platinum electrode has several important practical advantages over the well established mercury-pool electrode for the coulometric determination of uranium. The consecutive determination of iron(III) and uranium(VI), or plutonium(IV) and uranium(VI) can be carried out with the same working electrode in the same solution and the coulometric oxidation of uranium(IV) to uranium(VT) is practicable. The rate of stirring of the cell liquor is much less critical in the case of the platinum electrode. Two main problems had to be overcome before a practical procedure could be achieved; hydrogen evolution during the uranium(VI)-(IV) reduction had to be eliminated so that 100% current efficiency could be obtained for the desired reaction and electrode-surface poisoning phenomena had to be controlled so that reaction times could be kept reasonably short. It was found that selection of a hydrochloric acid base solution containing a small amount of bismuth(III) enabled hydrogen evolution to be avoided: also electrode-surface poisoning with this base solution was not particularly serious and could be maintained at a satisfactorily low level by occasionally anodizing the electrode in dilute sulphuric acid. Bismuth(III) forms a complex with chloride ions and its presence increases the hydrogen overvoltage at the working electrode: no visible deposit of bismuth metal forms on the electrode during the uranium reduction. Samples containing nitrate can be analysed provided sulphamic acid is added to this hydrochoric acid base solution.     

New approaches to reprocessing of oxide nuclear fuel

Dissolution of UO₂, U₃O₈, and solid solutions of actinides in UO₂ in subacid aqueous solutions (pH 0.9–1.4) of Fe(III) nitrate was studied. Complete dissolution of the oxides is attained at a molar ratio of ferric nitrate to uranium of 1.6. During this process actinides pass into the solution in the form of U(VI), Np(V), Pu(III), and Am(III). In the solutions obtained U(VI) is stable both at room temperature and at elevated temperatures (60 °C), and at high U concentrations (up to 300 mg mL^?1). Behavior of fission products corresponding to spent nuclear fuel of a WWER-1000 reactor in the process of dissolution the simulated spent nuclear fuel in ferric nitrate solutions was studied. Cs, Sr, Ba, Y, La, and Ce together with U pass quantitatively from the fuel into the solution, whereas Mo, Tc, and Ru remain in the resulting insoluble precipitate of basic Fe salt and do not pass into the solution. Nd, Zr, and Pd pass into the solution by approximately 50 %. The recovery of U or jointly U + Pu from the dissolution solution of the oxide nuclear fuel is performed by precipitation of their peroxides, which allows efficient separation of actinides from residues of fission products and iron.     

Effect of cobalt-60 gamma radiation on the determination of uranium(IV) in phosphoric acid solutions of uranium(iv) oxide

The effect of ⁶⁰Co γ-radiation on aerated and deaerated phosphoric acid solutions of uranium(IV) oxide (UO₂) was studied as a function of temperature, concentration of UO₂, and radiation dose rate. The effect was measured in terms of the radiolytic yield of uranium(VI),Gu_VI. For solutions of high initial UO₂ concentration, Gu(VI) is largest for the aerated solutions at 25°; it is lowest for the deaerated solutions at 140°. The Gu(VI) is lower for the solution of low initial UO₂ concentration than for any of the solutions of high initial UO₂ concentration. At the high starting UO₂ concentration, the initial Gu(VI) values are always higher than the succeeding values; this effect is attributed to the depletion of oxygen originally present in the solution. Gamma radiation causes an error in the determination of the stoichiometry of UO₂; the error is a function of the radiation dose. This error can be minimized by excluding oxygen from solutions of UO₂ and by keeping the initial UO₂ concentration as low as possible.     

Spectroscopic investigation of uranium sorption on soil surface using X-ray photoelectron spectroscopy

A study was carried out to understand the sorption of uranium (U) onto soil surface and identify the species of U on soil surface using X-Ray Photoelectron Spectroscopy (XPS). For the study soil was amended with uranyl nitrate and surface speciation study was carried out by investigating the energy region for U in spectrum. Analysis of spectrum revealed that U is present in U(VI) state. Deconvolution of XPS spectrum of U(VI) sorbed on soil surface revealed that U(VI) species such as, UO₂ ²⁺ and (UO₂)_x(OH) _y ^(2x?y)+ form complex with silanol, aluminol and goethite sites. The possible surface complexation is: ≡Al(OH)₂UO₂ ²⁺, ≡SiO₂UO₂, ≡SiO₂(UO₂)₃(OH)₅ and ≡Fe(OH)₂UO₂.     

Determination of oxygen in ternary uranium oxides by a gravimetric alkaline earth addition method

The applicability of a gravimetric method based on alkaline earth metal addition for the determination of oxygen in ternary uranium oxides of the type M—U—O (M=La, Ce and Th) is described. The oxide sample is mixed with MgO or Ba_2.8UO_5.8 and heated in air under suitable conditions. Because uranium is completely oxidized to the hexavalent state during the reaction, oxygen can be determined from the weight change. Oxygen in La_yU_1-y O_2+x is determined up to y = 0.8 with a standard deviation for x of ±0.006 with MgO. For Th_yU_1-y O_2+x, the value of x is determined with Ba_2.8UO_5.8 with a standard deviation of ±0.01 at y = 0.8. For Ce_yU_1-y O_2+x, the method can be applied only for low cerium concentrations where y = 0–0.2; the value for x with Ba_2.8UO_5.8 at y = 0.2 showed a standard deviation of ±0.002.     

Microwave-assisted dissolution of ceramic uranium dioxide in TBP–HNO3 complex

Microwave-assisted dissolution of ceramic uranium dioxide in tri-n-butyl phosphate (TBP)–HNO₃ complex was investigated. The research on dissolution of ceramic uranium dioxide in TBP–HNO₃ inclusion complex under microwave heating showed the efficiency of the use of this method. Nitric acid present in the inclusion complex participates both dissolution of UO₂, and oxidation of U(IV)–U(VI), the resulting UO₂(NO₃)₂ extracted with tri-n-butyl phosphate. Dissolution rate depends on both temperature of microwave dissolution process, and concentration of nitric acid present in the inclusion complex. The most intensive dissolution process is when the concentration of nitric acid ≥2 mol/L and the temperature of 120 °C. From the experimental data obtained by two kinetic models activation energies were calculated. At the average activation energy of UO₂ dissolution in TBP–HNO₃ complex equal 70 kJ/mol, and reaction order is close to one, i.e. the reaction takes place in an area close to kinetic.     

Spectrophotometric determination of the oxygen to uranium ratio in uranium oxides based on dissolution in sulphuric acid

The oxygen to uranium ratio in uranium oxides such as U(3)O(8), UO(2+x) powders and UO(2) fuel pellets has been determined by a new spectrophotometric method. The method can be used for determination of O/U ratio in UO(2) pellets and powders on a routine basis. In the described method, uranium oxides in the powder form are dissolved in 2M sulphuric acid containing a few drops of HF. The concentrations of U(IV) and U(VI) are directly determined by means of the absorbances of these species at different wavelengths. For determination of the O/U ratio in U(3)O(8) powder samples, 630 and 310 nm are the wavelengths chosen for U(IV) and U(VI), respectively. For UO(2+x) powder, where the O/U ratio lies between 2.04 to 2.15, U(IV) and U(VI) are determined at 630 and 300 nm respectively, whereas for UO(2) fuel pellets, where the O/U ratio is less than 2.01, 535 and 285 nm are used. The molar absorptivity of U(IV) at 630 and 535 nm is 21.4 and 6.8 l.mole(-1).cm(-1) and that of U(VI) at 310, 300 and 285 nm is 178.1, 278.6 and 585 l.mole(-1).cm(-1), respectively. Standard deviations of +/-0.002 O/U ratio units for pellets and +/-0.004 O/U ratio units for powders have been achieved.     

Indirect determination of uranium by the on-line reduction and fluorimetric detection of cerium(III).

A novel flow injection method has been developed for the indirect determination of uranium by the on-line reduction and subsequent fluorimetric detection of cerium(III). A sample solution containing uranium(VI), prepared as a sulfuric acid solution, was injected into a sulfuric acid carrier solution and passed through a column packed with metal bismuth to reduce uranium(VI) to uranium(IV). The sample solution was merged with a cerium(IV) solution to oxidize uranium(IV) to uranium(VI) and the cerium(III) generated was then monitored fluorimetricaly. The present method is free from interference from zirconium, lanthanides, and thorium, and has been successfully applied to the determination of uranium in monazite coupled with an anion-exchange separation in a sulfuric acid medium to eliminate iron(III). The sample throughput was 25 per hour and the lowest detectable concentration was 0.0042 mg l(-1).     

Synthesis and characterization of nanometric powders of UO2+x, (Th,U)O2+x and (La,U)O2+x

This paper describes a new way of preparing nanometric powders of uranium oxide, to fit the needs of studies on UO₂ oxidation, through the electrochemical reduction of U(VI) into U(IV). These powders can also be doped with radionuclides if necessary. The precipitation of oxides occurs in reducing and anoxic conditions. This original method makes it possible to synthesize nanometric UO₂ powders with a calibrated size, as well as the Th- and La-doped UO₂ powders with a predefined composition. The powder characterization by the X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron Microscopy shows the formation of spherical crystallites of UO_2+x, (Th,U)O_2+x and (La,U)O_2+x phases. The composition can be defined by the initial Th/(Th+U) and La/(La+U) ratios in solution and the particle size can be controlled by varying the pH.     

Synergistic extraction of uranium(VI) with 1-phenyl-3-methyl-4-(2′-chlorobenzoyl)-pyrazolone-5 and acidic organophosphorus reagents

A study of the synergistic extraction of uranium(VI) from sulphuric acid solution with 1-phenyl-3-methyl-4-(2-chlorobenzoyl)-pyrazolone-5 (PMCBP) together with di-(2-ethylhexyl)-phosphoric acid (HDEHP) and also mono-(2-ethylhexyl)-2-ethylhexyl-phosphate (HEHEHP) is described. The results suggest that the compositions of the extracted species is UO₂XHA₂ and UO₂X₂H₂A₂ respectively. Models for the extraction mechanism is also discussed.     

Extraction of plutonium(IV), uranium(VI) and some fission products by di-n-hexyl sulphoxide

The extraction of nitric acid, plutonium, uranium and fission products such as zirconium, ruthenium and europium has been investigated using di-n-hexyl sulphoxide in Solvesso-100. Results indicate that Pu(IV), U(VI), Zr(IV) and Ru NO(III) are extracted as disolvates, whereas Eu(III) is extracted as the trisolvate. The absorption spectra of the plutonium(IV) and uranium(VI) complexes extracted are similar to those of the species extracted by TBP which indicate the similarity of the species involved. Preliminary studies show that irradiated di-n-hexyl sulphoxide extracts zirconium to a smaller extent than irradiated TBP suggesting the use of long chain aliphatic sulphoxides as promising extractants for the recovery of plutonium in high radiation fields.     

Precipitation and purification of uranium from rock phosphate

This study was carried-out to leach uranium from rock phosphate using sulphuric acid in the presence of potassium chlorate as an oxidant and to investigate the relative purity of different forms of yellow cakes produced with ammonia, magnesia and sodium hydroxide as precipitants, as well as purification of the products with TBP and matching its impurity levels with specifications of the commercial products. Alpha-particle spectrometry was used for determination of activity concentration of uranium isotopes in rock phosphate, resulting phosphoric acid, and in different forms of the yellow cake. Likewise, atomic absorption spectroscopy was used for determination of impurities. On the average, the equivalent mass concentration of uranium was 119.38 ± 79.66 ppm (rock phosphate) and 57.85 ± 20.46 ppm (phosphoric acid) with corresponding low percent of dissolution (48 %) which is considered low. The isotopic ratio (²³⁴U:²³⁸U) in all stages of hydrometallurgical process was not much different from unity indicating lack of fractionation. Upon comparing the levels of impurities in different form of crude yellow cakes, it was found that the lowest levels were measured in hydrated trioxide (UO₃·xH₂O). This implies that saturated magnesia is least aggressive relative to other precipitants and gives relatively pure crude cake. Therefore, it was used as an index to judge the relative purity of other forms of yellow cakes by taking the respective elemental ratios. The levels of impurities (Fe, Zn, Mn, Cu, Ni, Cd and Pb) in the purified yellow cake were found comparable with those specified for commercial products.